I think maybe what they were saying was that they were not sure of what was causing it, or even certain it was occurring. Remember that they said they had three "thermometers" (really thermocouples) monitoring the temperature inside the pressure vessel, one showed an increase, the others didn't. That raises the question of an instrumentation problem with the one thermocouple. If it was a legitimate reading, the mechanism for the increase could be any number of things that are explainable by conventional heat transfer effects, things like blocked flow, buildup of vapor (which can inhibit conductive heat transfer), shifting of the heat-generating mass, deposition of insulating materials (debris) from the coolant flow, etc.
We all must admit it’s going to be very difficult to get someone to go in there and check eh!
Thanks for sharing - interesting discussion...